Understand the value of the enviroment variabe?

this line is assigned to an enviroment variable:

postgresql+psycopg2://airflow:airflow@postgres/airflow

And I do not really understand what it's mean.

My guess is that that is an SQLAlchemy database URL; see https://docs.sqlalchemy.org/en/20/core/engines.html

According to the above page generic syntax is:

dialect+driver://username:password@host:port/database

So for "postgresql+psycopg2://airflow:airflow@postgres/airflow"

• "postgresql" is the SQL dialect
• "psycopg2" is the database driver
• "airflow" is your database user name
• "airflow" is your database password
• "postgres" is your database hostname (!?!)
• "airflow" is the your database name

(The hostname "postgres" is not qualified, so it would presumably be resolved in your "local" DNS resolver.)

I searched it up in the bash documentation, but still cannot understand it correctly. What is the meaning of plus (+) in this case?

You were looking in the wrong place. The correct place to look would be the documentation for the product that you are attempting to install / configure.

A + in an environment variable's value has no special significance to bash.